This invention relates to current-fed inverter topologies and more particularly to the use of a field-effect transistor as a commutator circuit for a current-fed inverter.
Commutator circuits for SCR current-fed inverters are generally complex in that they include snubbers, LC circuits and other commutation components. For example, in a fullwave N-phase bridge circuit using thyristors SCRs or triacs), the thyristors are switched from on to off by first diverting the current through a shunt switch followed by reapplication of voltage when the shunt switch is turned off. That voltage must not exceed a critical rate of rise (dv/dt) or it will switch the thyristor back on, a phenomenon referred to as "forward breakover." Prior to the invention described in a copending application Ser. No. 187,620 filed Sept. 15, 1980, it was the practice to add a snubber (dv/dt control circuit) in parallel with the thyristor. The invention disclosed in that application employs gate-to-drain capacitance of a field-effect transistor (FET) for dv/dt control. That capacitance may be increased with an external capacitor, if required, without adding complexity to the FET commutator circuit. It is thus shown that an effective synergism results when FET and thyristor switches are combined within one topology.
Another type of switching device is the bijunction transistor (BJT) commonly referred to as a bipolar transistor, such as a silicon npn transistor. Compared with the FET, the strong points of the BJT include lower cost per volt-ampere and lower forward drop at full current. In contrast, the FET is superior in terms of speed, ease of drive, ruggedness, and overload capability. Accordingly, any potentially synergistic topology which combine FETs and BJTs should rely on the FET during the switching intervals, for reasons of both speed and ruggedness, while relying on the BJT during nontransient operation for low forward voltage drop.
Compared with the BJT, the SCR offers lower cost per VA, greater peak current capability, higher voltage and current ratings, easier drive and greater ruggedness. Relative weaknesses of the SCR include lack of inherent turn-off capability, limited switching speeds, and an on state voltage drop which is typically higher than BJT saturation voltages.
The question therefore arises whether or not effective synergisms can be realized when either FETs and BJTs or FETs and SCRs are combined in a new topology. A further question is whether or not the dv/dt control philosophy of the basic FET and SCR combination disclosed in the aforesaid copending application can be used to advantage in such new and useful topologies.